Saddle binding equipment for gathering signatures and binding them in book form is well known. See McCain U.S. Pat. No. 3,087,721. In these machines, signatures (which are folded sheets to be assembled in a book) are stacked in respective supply hoppers and are fed to a gathering chain moving past the hoppers, with one signature being dropped atop another. There may be as few as two active hoppers or as many as fifty or more hoppers, depending on the particular book being assembled.
As mentioned in McCain U.S. Pat. No. 3,589,712, it is necessary to keep the supply hoppers full with signatures. Further, it is necessary that the signatures be joggled or fanned or riffled prior to placing them in the supply hopper so they will not stick together as the result of friction, static electricity or fresh ink. This procedure is known as "breaking" the signatures. A sheet feeder for receiving freshly printed signatures, breaking them and transferring them to a supply hopper is shown in Bewersdorf U.S. Pat. No. 4,177,982. The disclosure of that patent is incorporated herein.
The supply station of the sheet feeder in the Bewersdorf patent is arranged to hold signatures in what is called a standing stack. The signatures in a standing stack lie in a generally vertical plane. That is, the signatures are placed edgewise on a generally horizontal support. It is desirable to load as many signatures as practical in the supply station of the sheet feeder so that the frequency of the loading operation may be minimized. Consequently, the supply station is made to be relatively large. Due to the essentially horizontal nature of the supply station, a relatively large amount of floor space is required to accommodate it.
In bindery shops where floor space is at a premium, it would be desirable to minimize the floor space required by the supply station by arranging it vertically as opposed to horizontally. In a vertical supply station the signatures would be arranged in a so-called flat stack, with one atop another. In other words, the signatures in a flat stack lie in a generally horizontal plane as opposed to the vertical plane of a standing stack.
A vertical supply station for a sheet feeder which breaks the signatures solves the floor space problem but leads to a different problem. In a flat stack or pile, most of the weight of signatures toward the top of the pile will bear on those near the bottom of the stack. Thus, a stripping mechanism for extracting signatures from the bottom of the pile will have to overcome not only friction, static electricity and sticking due to fresh ink, but also the weight of the upper signatures bearing down on the bottom one. These elements can combine to cause a condition known as "rolling."
As explained above, a signature is a folded sheet which has two portions called legs joined at a backbone. In a flat stack, the legs will be arranged one atop the other. The backbone will typically be leading. Rolling is defined as the condition resulting when the lowermost leg advances out of the pile under the influence of the stripper mechanism while the upper leg remains stuck to the adjacent signature in the pile. That is, the stripper mechanism will grab the lower leg and start moving it out of the stack. But the upper leg, instead of moving with the lower leg, remains stuck to the signature which is second from the bottom of the pile. Thus, when rolling occurs, the effect is an undesired unfolding of the signature. The signature legs tend to unfold or open up instead of moving out of the pile as a single folded unit.
Rolling is clearly an unacceptable condition for a sheet feeder. Although a flat stack is most prone to producing the rolling problem, it can also occur in a standing stack.